The increasing demand for platinum group metal use in industrial applications has made an impact on the industrial practice of recovering and refining precious methods. To meet this challenge, the classical precipitation techniques are being abandoned for more modern separation methods. Among the latter, solvent extraction (SX) has proven to be a suitable and powerful separation technique for the precious group metals.
Briefly, solvent extraction comprises two steps. In the first, the extraction step, dilute aqueous feed solution, containing the metal ion(s) to be recovered, is mixed with an immiscible hydrocarbon diluent or carrier containing a liquid ion exchanger or ligand dissolved therein, and the resulting metal chelate migrates to the organic phase. In the second, the stripping step, the separated "loaded" organic phase is mixed with an aqueous solution of a stripping agent (e.g., sulfuric acid) and the procedure is reversed, the metal ion passing back to the new aqueous phase. As a consequence, the dilute feed solution is converted into a highly concentrated solution, from which the metal values are more readily recovered, e.g., by electrolysis.
Despite, however, the superior performance of the SX process, these methods are not without their drawbacks The principal cause for their weaknesses is that the reagents used are not necessarily compatible with PGM-bearing feedstock solutions. In addition when the concentration of the desired metals is low, recovery and separation of Pt and Pd using SX methods are extremely difficult.
One way of overcoming these drawbacks is to employ an ion-exchange resin which contains a complexing or extracting agent. In this case, the desired precious metals are physically absorbed onto the resin and removal is accomplished by acid washing.
The compound 8-hydroxyquinoline is well known for its ability to coordinate with a variety of transition metal ions through covalent bonding to form a stable 5-membered ring via metal chelation. Recently, however, the ability of 8-hydroxyquinoline to form chelates with platinum group metals has been employed in SX of feed streams containing such precious metals. Unfortunately, this extracting agent is not sufficiently soluble in the hydrocarbon solvents employed in the first step of the SX process, and it is too soluble in the acidic aqueous stripping phases. The deficiencies of 8-hydroxyquinoline can be overcome by attaching 8-hydroxyquinoline to a resin backbone.
Resins developed by Bayer in U.S. Pat. Nos. 3,882,053 and 3,989,650 contain aminoethyl terminated cross linked polymers. In DE Patent No. 50153.00A, the aminoalkyl terminated cross linked polymers are further modified by condensation of these amines with 8-hydroxyquinoline and aldehydes. This reaction results in a chemical bond between the hydroxyquinoline and the resin. This chemically modified resin was then used to extract precious group metals from their acid chloride feeds. The said condensation discussed above is believed to generate additional chelating sites on the resin; however, it was determined that Pt could not be removed from the resin by normal water washing process.
A growing industrial concern involves the recovery of Pd, Pt and Rh from spent automotive catalytic converters. Because of the low levels of such precious metals and the high levels of contaminants found in the solutions obtained by leaching catalytic converters, prior art methods do not offer an efficient way of cleanly separating these metals. Additionally, these methods usually result in a lead-contaminated Pt solution. Thus, it would be extremely beneficial if a recovery process could be developed which can separate low levels of precious metals, i.e. platinum group metals, and eliminate contamination of the final solution.